Cement plugs are used in connection with cementing operations in a well for a variety of purposes, for example to provide interfaces between the upper and/or lower ends of a cement slurry column and the mud or displacement fluids that precede or follow it in the casing. When a predetermined volume of cement slurry has been displaced down into the casing, a plug can be used in a manner such that it separates the top of the column from the following displacement fluid. After substantially all of the cement has been pumped out of the casing and into the annulus between the casing and the well bore wall, a surface indication is needed so that pumping operations can be terminated at a point where most of the cement has been displaced into the annulus. Otherwise there is a risk of overdisplacement with attendant difficulties.
A high degree of skill, and occasional guesswork, is required during a cementing operation of the type described to determine when the plug has reached a certain downhole location in the casing. Various devices have been used, such as a simple casing nipple with an internal rubber sleeve that provides a reduced diameter in the bore, in an effort to create an observable pump pressure surge at the surface which indicates that the plug is passing through the bore of the nipple. However, this type of device has not, in the main, provided a reliable surface indication. One or more improvements over such devices are disclosed and claimed in my U.S. Pat. No. 4,907,649 issued Mar. 13, 1990, and in my U.S. application Ser. No. 678,007 filed Apr. 1, 1991. Although the inventions disclosed and claimed therein represent distinct advances in the art, there remains a continuing need for improvements which will provide a simple and reliable restriction sub that will create a distinctive, positive surface indication of the point in time when a displacement plug has reached a certain distance above the bottom of the casing.
A general object of the present invention is to provide a new and improved restriction sub apparatus which includes a uniquely arranged plastic sleeve member which impedes the passage of a cement displacement plug therethough in a manner such that a positive surface indication, as discussed above, is given.